(1) Field of the Invention
The present invention relates to a method for refining an electrical insulating oil and, more particularly, to a method for refining an electrical insulating oil by refining a heavy by-product oil produced in the preparation of ethylbenzene or ethyltoluene by alkylating benzene or toluene with ethylene in the presence of an alkylating catalyst, thereby producing an electrical insulating oil having excellent electrical characteristics.
(2) Description of the Prior Art
Production of a heavy by-product oil containing 1,1-diphenylethane in the preparation of ethylbenzene by introducing ethylene into benzene in the presence of an alkylating catalyst is known as described in, e.g., U.S. Pat. No. 4,111,825. This prior art reports that such a by-product oil is useful as an electrical insulating oil. The present inventors found that excellent electrical characteristics of a capacitor could not be obtained even if a fraction of 1,1-diphenylethane having a boiling point of 265 to 285.degree. C. and contained in the by-product oil was recovered and was impregnated in a polypropylene film to prepare an oil-filled capacitor due to impurities inevitably contained in the fraction. According to an analysis of the present inventors, the fraction contained alkyldecalins, cyclohexylethylbenzene, ethylcyclohexylbenzene, and the like in addition to 1,1-diphenylethane. These impurities tend to undesirably swell the polypropylene film. The fraction consists of a low aromatic group which tends to undesirably degrade electrical characteristics. These impurities have boiling points close to that of 1,1-diphenylethane and cannot be separated by a simple distillation method. If these impurities are eliminated and the oil can be refined by a method replacing distillation, an electrical insulating oil having excellent electrical characteristics can be provided.
U.S. Pat. No. 4,681,980 proposes as an electrical insulating oil an aromatic olefin-containing fraction produced by dehydrogenating a diarylalkane-containing fraction as a by-product oil in the preparation of ethylbenzene. Hydrogenation is not performed although dehydrogenation is performed in the above patent. Therefore, the method described in U.S. Pat. No. 4,681,980 is not a method for refining the diarylethane-containing fraction.